Cancer is among the leading causes of death worldwide. Resistance to chemotherapy and molecularly targeted therapies is a major problem facing current cancer research and treatment. As an example, acute myeloid leukemia (AML) is a clonal hematologic malignancy with great variability in the clinical features, pathogenesis and treatment outcomes. This malignant disorder is caused by abnormal differentiation of hematopoietic precursor cells forfeiting their ability to respond to regulators of proliferation. The standard therapeutic approach for AML patients is initial chemotherapy induction followed by post-remission treatment, with additional chemotherapy cycles or allogeneic stem cell transplantation for relapse prevention (1). Although significant progress has been achieved, current treatments for AML may only offer limited survival benefits rather than provide fully satisfactory responses, presumably due to chemoresistance and disease relapse (1,2). To reduce recurrence rate and promote therapeutic efficacy, it is urgent to identify new targets (3,4).
Nuclear factor-κB (NF-κB) controls various aspects of immune responses and superiorly regulates cell survival, proliferation, and differentiation (5). Recent evidences attribute a growing number of malignancies to aberrant activation of NF-κB that cross-talks with other signaling molecules and pathways (6), thus it is considered a risk factor for poor prognosis in several types of cancer including leukemia (7). In agreement, constitutively activated NF-κB was shown to protect tumor cells from apoptotic stimuli and promote their resistance to chemotherapies and ionizing radiation (8), presumably through transcriptional activation of anti-apoptotic/pro-survival factors Bcl-2 and Bcl-XL (9). In line with this notion, the role of NF-κB in leukemogenesis has also been addressed for AML (10), and the inhibition of NF-κB re-attains chemosensitivity in this hematopoietic malignancy presumably due to attenuated pro-survival responses and activated pro-apoptotic signals (11). These observations collectively point to the NF-κB signaling axis as a promising therapeutic target (12).
The Aurora family of serine/threonine kinases promotes tumor proliferation through regulation of chromosome alignment, segregation, and cytokinesis during mitosis (13), and has been suggested as anti-cancer target (14). Upregulation of Aurora A has been reported for bone marrow (BM) mononuclear cells in AML patients (15), and shown to be associated with unfavorable-risk cytogenetics and higher white blood cell (WBC) counts (16). In addition, Aurora A has been shown to promote in vitro and in vivo chemoresistance of cancer cells by reducing chemotherapy-induced apoptosis through activation of NF-κB signaling pathways (17). In support, an increasing ratio of Bax/Bcl-2, as suggestive of promoted apoptosis, has been reported upon treatment with Aurora A inhibitor VX680 (18), a drug that showed clinical effectiveness to chronic myeloid leukemia (CML) (19).
TIFA, a TRAF-interacting protein, is a relatively new player in the NF-κB signaling pathway. Our previous study uncovered that overexpression of TIFA is able to promotes NF-κB activity in a TNF-α dependent manner, and that silencing of TIFA attenuates this TNF-α-stimulatedd NF-κB signaling (20). Mechanistically, this signal axis is initiated by a TNF-α-dependent phosphorylation of TIFA at threonine 9 (pThr9) that interacts with the forkhead-associated (FHA) domain of TIFA to facilitate oligomerization of TIFA dimer (20,21). TIFA oligomerization in turn supports the high-ordered architecture of TRAF family components, such as TNF receptor associated factor 2 (TRAF2) or TRAF6, and modulates ubiquitination of TRAF6 to activate IκB kinase (IκK) complex, through which IκB is subsequently phosphorylated and undergoes ubiquitination-dependent degradation allowing nuclear translocation of NF-κB to transactivate downstream factors (22). In addition to TNF-α stimulation, it was shown that such phosphorylation-dependent oligomerization of TIFA is also triggered by a Gram-negative bacteria derived monosaccharide heptose-1,7-bisphosphate (HBP) to activate innate immunity (23), and that TIFA mediates innate immune response through assembly of NLRP3 inflammasome upon endothelial sheer stress (24), suggesting an essential role of TIFA in modulation of innate immune responses and inflammation.
The FHA domain is composed of around 80-120 amino acids and known to recognize phosphothreonine (pThr) specifically to exert signaling function. The sequence homology among different FHA-containing proteins is relatively low, however the overall structural architecture of FHA domains is well conserved, in which two β-stranded β-sheets forms a β-sandwich. The β-strands are connected by loops which, though varying greatly in length, are responsible for recognition of the specific pThr-ligand. The FHA-pThr binding has been shown to regulate diverse biological functions, ranging from DNA damage repair, cell cycle checkpoints, to signal transduction. The specificity, biological function, structure, and mechanism of FHA domains have been summarized in recent review (25).